Lightbox2 trigger for class="image-img_assist_custom" ?

I just switched to Lightbox2 from Thickbox to work in conjunction with the Image module. I've been pleased, especially give the fact that Lightbox2 can now handle showing the original image when the bigger sizes aren't available.

I like that I can set images of the Thumbnail size to trigger Lightbox2. However, in many nodes, the images that are embedded (using Image Assist) are not Thumbnail size but a custom size. It seems that in Lightbox2, there is no setting to make these images trigger a Lightbox to pop up. I believe Thickbox can be set to be used on images of a particular class. Is there a way to do this in Lightbox2?

Thanks for any help.